WorkflowStub::startUpdate() sent an empty update id and relied on the
Temporal frontend to assign one. The frontend does; the Java time-skipping
test server does not — the empty id reaches the worker, which panics
("update id is empty, can't validate update"), failing the update workflow
task in a retry loop and wedging the whole execution. The update result
never returns and subsequent signals stop being processed (issue #577).

Generate the id client-side via Common\Uuid::v4() when the caller did not
supply one, matching sdk-go, sdk-java and sdk-typescript.

Adds a functional regression test that runs an update against the
time-skipping test server (WithoutTimeSkipping).

Closes #577